Communication units are well known in the art. The communication unit may comprise a telephone set. The telephone sets may be either a wireless telephone sets or a wireline telephone sets. The wireless telephone sets may be either portable or mobile telephone sets. The wireless telephone sets may be either cellular or cordless telephone sets. The telephone sets typically transmit audio signals via a microphone and receive audio signals via a loudspeaker. The audio signals may be either analog or digital audio signals. The telephone set may comprises either a "hands free" telephone set or a telephone hand set. With the hands free telephone set, the microphone and the loudspeaker are located in close proximity to the user to permit the user to engage in a telephone conversation without holding the telephone hand set. With the hands free telephone set, the microphone and the loudspeaker may be located in the same enclosure or separate enclosures. With the hands free telephone set, the user's "hands" are "free" to use for other purposes and a plurality of user may engage in the same conversation. With the telephone hand set, the microphone and the loudspeaker are located in the same enclosure such that the loudspeaker and the microphone may be substantially aligned with the user's ear and mouth, respectively, when the user holds the enclosure next to his head. The telephone handset permits the user to engage in a telephone conversation while holding the telephone hand set in one hand. Although, with the telephone hand set, the user's hand are not "free" to use for other purposes, the close coupling of the loudspeaker and the microphone with the user's ear and mouth reduces background noise and increases privacy.
A well known problem associated with communication units is acoustic feedback. This problem particularly manifests itself in hands free telephone sets. In hands free telephone sets processing analog signals, the occurrence of the acoustic feedback problem is not very frequent. This is because the acoustic feedback sounds like sidetone and frequently goes unnoticed. However, even when processing analog signals, if too much acoustic feedback enters the system, the system can become unstable, which results in "howling". In hands free telephone sets processing digital signals, not much feedback can be tolerated. In hands free telephone sets processing digital signals, the time required to process the digital signals produces a large time delay between the transmitted voice of the other party and the acoustically fed back received voice of the other party. The large time delay causes the acoustic feedback to be perceived as an echo which makes it difficult for the other party to communicate with the user of the hands free set. Therefore, the hands free telephone sets processing digital signals need less acoustic feedback than can be tolerated in the hands free telephone sets processing analog signals.
In a typical hands free telephone set, the loudspeaker and the microphone are connected through appropriate amplifying circuitry to a transmission path which is communicatively connected to a public switch telephone network. A voice signal received from the transmission path is directed through an amplifier to the loudspeaker which transforms the signal from electrical to acoustical energy and permits the signal to be heard at a distance from the loudspeaker. A voice signal received by the microphone is typically amplified by an amplifier and transmitted to another telephone set participating in the conversation through the transmission path provided by the public switched telephone network.
Because the microphone in many hands free telephone sets is located in proximity to the loudspeaker, the signal received over the loudspeaker is often acoustically coupled back into the microphone and returned along the transmission path to the other telephone set. The user of the telephone set can often hear the return of the acoustically coupled signal, which sounds similar to an echo, and is distracted and annoyed thereby.
Many telephone systems utilize a single line for the transmission of telephone signals in both directions. In each single line system, the acoustically cross-coupled signal which is fed back into the microphone at the hands free instrument will couple through the single transmission line or through a hybrid circuit which connects the telephone set to the single line back to the loudspeaker of the hands free telephone set. If the amplification of the received and transmitted signals is of a sufficiently high level, the acoustically cross-coupled signal which is subsequently electrically cross-coupled on the transmission path can result in an increasing amplitude signal running around the acoustic and electrical coupling means until the amplifiers of the hands-free telephone set are saturated. Such a signal often causes an extremely annoying squeal in the telephone set.
The acoustically cross-coupled signal is also annoying because it may mix and interfere with any signals originating at the hands free telephone set. The recipient of such a mixed signal may have considerable difficulty in separating the original signal from the acoustically cross-coupled signal, both of which are received by the same microphone input.
There have been a number of methods and circuits proposed to reduce and/or eliminate acoustic and electrical coupling in hands free telephone sets. In one known method a hands free telephone set is equipped with an input detection device which operates to disable the transmission sections of the telephone set when a signal is received from the transmission path. Accordingly, when the user of the telephone set hears a communication from the party with which he is speaking, he is unable to respond until the other party ceases speaking.
Similarly, in other known systems, the input detection device is also used to control the receiver circuitry, so that when a local input is received through the microphone of the telephone set, the receiver circuitry is disabled. Accordingly, in such systems it is a frequent occurrence that the user of a hands-free telephone set is unable to hear from the other telephone set while he is speaking.
While the technique of disabling one of the circuits in a telephone set may be effective in eliminating or reducing feedback, such a system is generally disadvantaged by the fact that, at any one time, only one party to the communication can transmit a signal which is received by the party, i.e., one party cannot interrupt the other party until the other party has finished his communication or if one party begins to communicate, he can no longer hear the other party.
A refinement to systems which simply disconnect one or both of the signal paths are systems in which the transmission path is disabled only upon receipt of a signal of predetermined strength on the receive path. While such devices are generally improvements of the simple disabling devices, they are, nonetheless, limited by their disabling of one of the communication paths.
It is also known to remove or eliminate echo signals from telephone transmissions by use of digital echo cancellers, sometimes called "adaptive filters". In such systems the signal being transmitted by the telephone set is compared with the signal received from the transmission line within a predetermined time period and if the two signals are substantially similar as determined by a digital filter, the received signal is subtracted from the signal to be transmitted prior to amplification and transmission. While echo cancellers reduce the echo/feedback problems in the hands-free telephone sets, they are generally complex electronic circuits and often require a relatively long period of time for the digital filter to converge and become effective.
Other systems disclosed, employ variable amplification circuitry in the communication receive path of the telephone set. In such known systems, often a measure of the signal level in the transmit path of the telephone set is taken and used to control the gain of the receive path amplifier in an inverse relationship, i.e., when a high volume signal is transmitted by the telephone set, the amplifier circuit in the receive circuitry is adjusted to decrease the amount of amplification applied to the signals received at the telephone set. In such systems, however, the gain of the amplifier on the receive circuits in the telephone set depends in large part upon an independent signal, the signal to be transmitted, and the receive signal may be unduly suppressed.
Accordingly, there is a need for an audio signal processor and method therefor for substantially reducing audio feedback in a communication unit.